Plants and apparatus of the type in object have the task of removing fluids from the patient's mouth during a dental operation. The fluids comprise a gassy part, usually air, and a liquid part, generally water and other liquids used in the dental apparatus, but there is also a solid part mostly containing particles of dental amalgam used in filling.
Many prior-art dental aspiration plants include a separator chamber in which a depression is created, for example by means of a suction pump. The tube aspirating the fluids from the patient's mouth is connected to the separator chamber, where the fluids are borne and deposited. These fluids are then discharged from the separator and sent to the sewers, while the air is extracted from the separator by the suction pump.
In some known methods, the liquids are extracted from the separator by a drainage pump which removes the liquids from the separator chamber--which is in depression--and discharges them into the environment at atmospheric pressure, for example into the municipal sewers.
A device of the above-described type is described and illustrated in European Patent EP 211808.
Downstream of the drainage pump there is usually a single-acting valve allowing only pump-to-environment liquid flow at atmospheric pressure, and closing the discharge pipe when the pump is not operating.
The single-acting valve, which is normally of the oscillating obturator type, can at times function faultily due to the fact that the liquids being extracted by the pump and discharged into the sewers contain solid particles (amalgam, encrustations, various residues) which can get lodged between the valve obturator and its seating and prevent the valve from closing properly when the pump is stopped. This can cause a back-flow of air into the discharge pipe due to the difference in pressure between the external atmosphere, kept at atmospheric pressure, and the separator chamber, which is in depression, with a possible consequent emptying of the liquid in the pump internal pipes. Lack of liquid can lead to considerable pump restart difficulties, and has led to pump design which allows for the possibility of restart even in the absence of liquids. These pumps, however, are expensive to build, run and maintain.